Hormonal contraceptives, which are used by women around the globe, contain female hormones which prevent ovulation (the release of an egg). A new study suggests this highly reliable method of birth control may pose an unseen danger. Women who use birth control pills, the patch, or other hormonal contraceptives for at least five years may be increasing their risk of developing a rare tumor of the brain, Danish researchers say.

“It is important to keep this apparent increase in risk in context,” says Dr David Gaist of Odense University Hospital and University of Southern Denmark. “In a population of women in the reproductive age, including those who use hormonal contraceptives, you would anticipate seeing 5 in 100,000 people develop a glioma annually, according to the nationwide Danish Cancer Registry.”

How do women decide which contraceptive method to use? This seemingly simple choice can spin out of control when an already complex selection process becomes complicated by a natural desire, many would say need, for convenience. (After all, the only birth control method that works is the one that is used.) And so it is convenience that often persuades a woman to accept hormonal contraceptive. After all, not only are they reliable but they come in a variety of forms, including the pill and the patch, a simple shot, an implant, a vaginal ring, and a hormonal IUD.

Though won over by one (or some) of these birth control products, many women still continue to wonder, Is it healthy to take hormones? Unfortunately, this is not easy to answer. Some scientific evidence suggests hormones increase the risk of certain cancers, but then other studies indicate the use of contraceptives may reduce the risk of cancer in certain age groups. In short, it’s complicated.

For the current study, then, researchers decided to pinpoint just one cancer, glioma (a rare brain tumor), and one age group, premenopausal women. The team dipped into Denmark's national administrative and health registries, which enabled them to identify any woman, between age 15 and 49, who had been diagnosed with glioma during the years 2000 through 2009. After discovering 317 cases in the database, they compared each of these women with eight age-and-education matched women without gliomas, a total of 2,126 “controls.”

“A nearly two-fold increased risk of glioma was observed among long term users of hormonal contraceptives,” the authors wrote in their study. Among younger women who had ever used hormonal contraceptives — defined as two or more filled prescriptions for most methods — they found “a moderately increased risk of glioma.”

Source: Andersen L, Friis S, Hallas J, Ravn P, Kristensen B, Gaist D. Hormonal contraceptive use and risk of glioma among younger women: a nationwide case-control study. BJCP. 2014.